[mlir] Don't assert when simplifying certain `AffineExpr`s (#78855)

Currently, `simplifyMul()` asserts that either `lhs` or `rhs` is
symbolic or constant. This method is called by the overloaded `*`
operator for `AffineExpr`s which leads to a crash when building a
multiplication expression where neither operand is symbolic or constant.
This patch returns a `nullptr` from `simplifyMul()` to signal that the
expression could not be simplified instead.

Fix https://github.com/llvm/llvm-project/issues/75770